1. Field of the Invention
The present invention is related to antenna systems and, more particularly, to a method and system for mitigating static mode multipath effects, such as errors in carrier phase, code, and signal noise.
2. Description of the Prior Art
The corrupting effect of multipath signals on carrier phase measurements, signal noise, and code data obtained with an antenna system is one of the limitations to achieving high accuracy positions in a wide variety of applications. The problem is especially a concern for GPS reference stations whereby the static environment may induce slowly changing specular effects which do not easily average out.
One conventional approach to mitigating the resulting errors is by modifying the antenna gain pattern, such as incorporating a choke ring with a ground plane, to produce a cutoff near the horizon and counter the presence of multipath signals. Another approach, which utilizes an antenna designed to have sharp cutoff below a certain elevation angle, is disclosed by C. Bartone and F. van Graas in Proceedings of IEEE PLANS, Airport Pseudolites for Local Area Augmentation, 1998, pp. 479-86.
However, even with antenna systems having sharp cutoffs below an elevation of 15.degree., multipath signals from tall structures would still pose a problem. Moreover, for GPS applications, such a cutoff reduces the available coverage of the antenna and compromises the operation of the GPS system. In the alternative, there are various methods to mitigate code multipath by using a multiple signal classification technique with multiple antennas and an extended multipath estimation delay lock loop, as disclosed by D. Moelker in Proceedings of ION GPS-97, 1997, Multiple Antennas for Advanced GNSS Multipath Mitigation and Multipath Direction Finding, pp. 541-50.
However, there still remains a need for an improved apparatus and method of mitigating multipath-induced errors in satellite signal reception, and it is an object of the present invention to provide such an apparatus and method.